The present invention is related to spread spectrum communication systems and, more particularly, to spread spectrum communication systems having a plurality of transmitters at remote locations capable of transmitting information to one or more receivers.
The Federal Communications Commission provides for use of low-power communication devices under Part 15 of its rules with the purpose of permitting such use of these communication devices under less stringent requirements than are typically found in other Parts of its rules, such as permitting unlicensed operation, to reduce cost and to promote providing to the public the benefits that come with increased communication device use. That the Commission provides this opportunity follows in large part from the perceived reduced likelihood of interference by a user of such a device with other users of that and nearby portions of the electromagnetic spectrum because of the low values of radiated power permitted in the transmitting of electromagnetic radiation in the permitted frequency bands.
These rules have been recently amended to now permit the use of two kinds of spread spectrum systems thereunder, direct sequence systems and frequency hopping systems. Such spread spectrum systems reduce power density of the electromagnetic radiation transmitted thereby at any frequency or narrow band of frequencies within the total frequency bandwidth being used in such transmission which thus reduces the probability of that radiation interfering with other transmissions in the same total frequency bandwidth. Further, spread spectrum systems tend to be able to tolerate strong interference from other radiation in the total frequency bandwidth it is using without that interference causing disruption of the operation of such a system. These characteristics thus conform well to the purpose of the Part 15 rules and the Commission, as a result, has authorized relatively high output powers for such spread spectrum systems compared to the other low-power devices authorized under those rules.
One desirable use for such spread spectrum systems under the Part 15 rules is in the implementation of security systems. Typically, such a system will have a number of locations in which some change of condition from the normal situation is to be sensed, this sensing information being sent to a transmitter which in turn transmits that information to a central receiver, or receivers. Thus, the opening or closing of an access limiting means at an opening in a wall, such as a door or window, might be sensed, or the motion of a body in some monitored space or spaces might instead be sensed as a basis for securing a building or other facility against entry by intruders. Alternatively, changes in constituents of the atmosphere surrounding a sensor might be sensed such as the onset of smoke or other chemical change, or the temperature, or light levels, or the like, might instead be sensed. The information sensed, and perhaps other information provided in connection therewith by other sensing devices or some other means, is passed to the nearest transmitter or transmitters in the spread spectrum system for transmission to the desired receiver.
Difficulties in such transmissions between remotely located transmitters at various sensing stations in the field and the central receiver, or receivers, can easily arise. Often, such security systems are desired to operate in facilities of substantial value which usually implies that a number of structures or structural portions will often be present in the vicinity of the transmission path between such transmitters and the desired receiver or receivers. Reflection of the transmitted electromagnetic waves will result in increasing the number of alternative transmission, or radiation, paths between each of the transmitters and a receiver so that various portions of the electromagnetic waves reaching the receiver from a transmitter will have followed different paths. As a result, there can easily be cancellation at the receiver of the electromagnetic radiation transmitted by a transmitter at some frequencies.
Another problem which arises when the number of remotely located transmitters are able to transmit information over electromagnetic waves to a central receiver at any time is the possibility of two or more transmitters simultaneously transmitting, and thereby interfering with one another. Either of the foregoing problems could lead to preventing communication between a transmitter and the receiver or receivers. This is a serious difficulty since the reliability of communication between any one transmitter and a central receiver must be quite high in a security system if the user of this system is to be alerted quickly to any changes in the conditions being monitored by that system.
Another consideration, especially for the remotely located transmitters at various sensing stations, is minimization of the use of electrical power. Such transmitters will, in at least some circumstances, be required to operate on battery power because of the nature of the facilities occurring at the sensing station at which it is located.
Thus, there is desired a spread spectrum communication system which can operate in the presence of multiple alternative transmission paths between the transmitters used therein and a central receiver or receivers, and which can tolerate at least some overlapping of transmissions from two or more of its transmitters to a central receiver or receivers. Further, such a system must operate with its transmitters, and perhaps receiver or receivers, drawing sufficiently low power while operating within the rules set forth by the Federal Communications Commission for low-power communication devices under Part 15.